Gas chromatography (GC) is an analytical method that measures the content of various components in a sample. The method for separating chemical substances relies on differences in partitioning behavior between a flowing mobile phase (gas phase) and a stationary phase supported in a column to separate the components in a mixture. As the gas flow passes through the column, the sample components move at velocities that are influenced by the degree of interaction of each component with the stationary phase in the column. Consequently, the different components separate in time as the components elute from the column.
While GC is widely used to resolve a mixture into its various components according to retention profiles of the different molecules passing through the GC column, and can potentially handle mixtures containing large numbers (hundreds, for instance) of substances, identifying the molecules that elute from the column is more problematic. For example, full peak separation is often needed to qualify and quantify the compounds present. Small sample sizes and dynamic ranges, and the need for continuing calibration are additional drawbacks.
To address the need for rapid and sensitive identification of the molecular species present, GC has been integrated with techniques such as mass spectrometry (MS) or Fourier transform infrared (FTIR) spectrometry.
Gas chromatography-mass spectrometry (GC-MS) is probably the most widespread tandem technique in the analytical instrumentation industry today. GC-MS systems are versatile and are employed across many different industries, particularly for environmental, chemical, petroleum, pharmaceutical, and toxicological applications. While GC-MS is a fast, sensitive technique suitable for multiple component detection and spectral identification, capable of measuring atomic species and supported by large available spectral libraries, it suffers from some disadvantages. These include compound separation to prevent MS interferences, non-linear calibrations, poor precision and accuracy (requiring constant calibration) and limited dynamic range. Problems also are encountered when high concentrations are present that can allow for chemical ionization to occur, generating questionable data.
While GC-MS is the more commonly deployed solution, Gas Chromatography-Fourier Transform Infrared Spectrometry (GC-FTIR) provides a powerful analytical tool that is particularly useful to distinguish among structural isomers that have identical electron impact and chemical ionization mass spectra.
Nevertheless, historically the designs of GC-FTIR systems have been plagued with their own limitations. For example, many GC-FTIR sample cells utilize a “light pipe” (typically a cell or cuvette used for passing both gas eluted from the GC column, and light from the FTIR interferometer). The light pipe is made relatively short to prevent peak dilution through the IR cell and its eventual IR detection or secondary detection. Since IR absorption is proportional to cell path length, this short path length limits the sensitivity (minimum detection limit (MDL)) of the technique. Problems also arise in cases in which GC peaks come off very quickly. Since the light pipe has a relatively large volume when compared to the flow rates of the GC, the gas can become diluted, making measurements more difficult.
More recently, Spartz, et al., in U.S. Pat. Appl. Pub. No. US 2015/0260695 A1, now U.S. Pat. No. 9,606,088, issued Mar. 28, 2017, both of which are incorporated herein by this reference in their entirety, disclose GC-FTIR techniques and systems with the objective of coupling existing or newly developed approaches, such as GCs, and/or optical spectroscopy systems (e.g., FTIRs) in ways that reduce or minimize the deficiencies encountered with conventional arrangements. According to U.S. Pat. No. 9,606,088, FTIR techniques, typically implemented by software executed by a computer are employed to identify unknown compounds present in a sample. The techniques can be applied or adapted to any analysis process or instrumentation that uses a FTIR spectrometry system or another suitable spectrometry system. In many cases, the spectroscopic analysis is further enhanced or facilitated by a temporal separation of unknown compounds present in the sample being analyzed. In specific implementations, the separator is a GC.
U.S. patent application Ser. No. 15/335,618, filed on Oct. 27, 2016, published on May 4, 2017 as U.S. Patent Application Publication No. 2017/0122920 A1, and incorporated herein by this reference in its entirety, outlines, in more detail, a switching systems upstream and downstream of thermal desorption tubes, the GC, and the spectrometry (e.g., FTIR) system sample cell. Arrangements and techniques disclosed in this document include, for example, a spectrometry system for detecting components of a sample; a gas chromatography column for separating the components of a sample; a first sample unit for receiving a first sample from a sample source; and a second sample unit for receiving a second sample from a sample source. Each sample loop unit allows independent processing of samples in preparation for analysis.